In their paranormal state, ghosts are unable to have much physical influence. They are visible, and appear as a greyish-silver apparition of their former (living) selves. They pass through solid objects without damaging themselves or the material, but create disturbances in water, fire and air. The temperature drops in the immediate vicinity of a ghost. Their appearance can also turn flames blue.[1]

In passing through an object, they may impart a sensation of icy coldness. However, they are apparently moved by gusts of wind, as demonstrated when a petrified Nearly Headless Nick was wafted to the Hospital Wing by a large fan, although this may have been possible due to his petrified state. Ghosts are either capable of, or pretend to be capable of, almost being able to taste rotten food.

Ghosts also appear to celebrate their deathday, as Nearly Headless Nick did so on October 31st in 1992. They tend to be very sensitive concerning the circumstances of their death, often showing great hesitancy when asked by others how they died.

They are also weightless and may fly in any direction. Ghosts cannot be destroyed, however, they may be petrified with the gaze of a Basilisk. This can be reversed by a Mandrake Restorative Draught, although the method of administration is unclear.

As ghosts are imprints of souls of the said deceased wizards and witches, they are unable to move beyond the veil, and are forever[citation needed] trapped in the world of living as intangible beings. Due to this, few would choose this path. Depending on the conditions of the person's soul, they may not be able to return, as Tom Riddle's self-mutilated soul is trapped in limbo for eternity and cannot become a ghost.

Each House at Hogwarts has a patron ghost. Each of these ghosts once belonged to their corresponding House. They apparently serve as a representative to their House, as well as serving the role of messenger or guide to those who are still unfamiliar with the House they were sorted into. It is unknown for how long they may retain this title.

The Gryffindor house ghost is Nearly Headless Nick. He is known as a friendly ghost who was nearly beheaded, but the job was poorly executed, which was how he obtained the nickname of "Nearly Headless Nick". He prefers to be addressed as "Sir Nicholas". His real name is Sir Nicholas De Mimsy-Porpington.

The Ravenclaw house ghost is the Grey Lady, also known as Helena Ravenclaw. She was the daughter of Rowena Ravenclaw, making her the only house ghost to be directly related to one of the four Hogwarts founders. She is renowned for being instrumental in assisting Harry Potter in the search for the Horcruxes.

The Slytherin house ghost is the Bloody Baron. He is well known for being extremely unsocial and many students, including those of his own House, are known to be slightly afraid of him. The Baron is also the only one known to be able to control Peeves. The Bloody Baron is responsible for the death of Helena Ravenclaw as well.

It is possible that while ghosts might be silver-greyish to the living, it is possible that through their eyes, they see themselves and each other the way they looked alive, as Nearly Headless Nick was able to read a Ghost letter that appeared to be transparent to Harry, meaning that he could not see any written text on it, even if Sir Nicholas could.

Although Killing rips the soul, true remorse can heal it again. One example would be the case of The Bloody Baron as he killed Helena Ravenclaw, but felt remorse over her death, thus he was able to come back as a ghost.